Switch test runner

[fb55]

We discussed this in #362 (review). Jest works alright as a test runner for now, but I'm happy to switch should we agree on an alternative.

[43081j]

we could just use mocha and c8? or uvu and c8?

uvu has pretty poor reporting unfortunately (no CI integration). in the past i've made use of its assertion library though paired with mocha for better reporting (i.e. so you can get test results etc in CI).

since the new version of parse5 might also work out of the box in browsers, it may be worth having a small suite of tests that ensure it runs there. via puppeteer or web-test-runner.